The present invention relates to waveguide structures including a region having a photonic band structure. Waveguide structures of this type can be used for a number of applications including lasers, filters, couplers and multiplexers.
Planar waveguide structures including regions having a photonic band structure are known in the art and have been used for the construction of waveguides and integrated optical circuits. The photonic band structures can be provided by forming a lattice of holes in a dielectric substrate, the geometry of the lattice of holes and the properties of the dielectric material determining the photonic band structure. WO 98/53351 (BTG International Limited) describes planar photonic band structures of this type and describes methods of producing them. Similarly, photonic band structures can be formed by an array of rods surrounded by air or another medium.
Particular geometries for the lattice of holes are chosen to produce particular effects. WO 01/77726 (BTG International Limited) describes quasi periodic geometries for the lattice of holes which exhibit high orders of symmetry.
Photonic band structures can also be used in optical fibres. The fibre has a regular lattice of air cores running along its length and transmits a wide range of wavelengths without suffering from dispersion. It is made by packing a series of hollow glass capillary tubes around a solid glass core that runs through the centre. This structure is then heated and stretched to create a long fibre that is only a few microns in diameter.
According to the present invention, an optical device comprises a waveguide structure having a photonic band structure region, the photonic band structure region including a first region having a first refractive index and an array of sub-regions having a second refractive index, the array of sub-regions being arranged In a Fibonacci spiral patter.
A Fibonacci spiral pattern can be found in nature in the arrangement of the seeds of a sunflower, florets of a cauliflower, pine cones and even in the shell of a Nautilus. In the context of the present invention, a Fibonacci spiral pattern is an optimal packing system for the sub-regions surrounding a central region.
The diffraction patterns formed by the array of sub-regions within the photonic band structure region are circular Bragg rings and this provides highly isotropic behaviour about a central region. The fact that the band structure is identical in all directions minimises the tolerance in placing input and output waveguides or fibres. This isotropy provides several benefits such as relaxed fabrication tolerances. Similarly, mode divergence from the end of a waveguide or fibre is not critical using the optical device of the present invention. The band structure also possesses complete bandgaps for TE and TM polarisations even for relatively low dielectric contrasts. These two features make it extremely useful in laser design. When, the first region is made of a lasing material, the structure exhibits an isotropic bandgap inhibiting emission in a particular wavelength range. The bandgap may include the stronger emission lines of the lasing material and so the structure can be used to suppress these spectral wavelengths and enhance other weaker spectral wavelengths.
Preferably, in a Cartesian coordinate system, the Fibonacci spiral pattern is defined as xn=cos(nxcfx86)n and yn=sin(nxcfx86)n where xcfx86=xcfx80(5xe2x88x921), and where n is the integer index for a point in the pattern. To generate the pattern a point is plotted for each value of n. Those values may be n=1, 2, 3,4 . . . etc. Alternatively, certain values of n may selectively omitted to create defects, ring patterns or zone plates. For example, odd values for n may be omitted leaving n=4, 6, 8, 10 . . . etc.
In one implementation of the present invention, the optical device is a planar device including a substrate, a buffer layer, a core layer and a cladding layer, wherein the core layer is sandwiched between the buffer layer and the cladding layer.
Preferably, the array of sub-regions are holes formed In the first region. Preferably, the holes are formed through the cladding and core layers. Preferably, the holes are filled with a material having a third refractive index.
The optical device may be a laser device including an active core layer. The active material may be a doped dielectric material such as erbium doped tantalum pentoxide, silicon nitride, silicon oxynitride or a lasing material such as gallium arsenide or indium phosphide. Preferably, the array of holes are formed in a lasing cavity.
Preferably, the core is made from a lasing material and the photonic band structure region has a photonic bandgap covering at least one lasing wavelength of the core. The photonic band structure region may be optically pumped so as stimulate lasing. Accordingly, the device may further include an optical pump source coupled to the photonic band structure region for stimulating lasing. Alternatively, an electric current may be passed through the photonic band structure region or through the core region so as to stimulate lasing in which case the device may further include an electric current source coupled to the photonic band structure region for stimulating lasing.
The optical device may be a vertical cavity surface emitting laser (VCSEL), in which the photonic band structure prohibits lasing in the plane of the spiral pattern and so emits a laser beam from a central cavity of the Fibonacci spiral pattern, perpendicular to the spiral pattern.
Alternatively, the optical device could be a filtering device. In one embodiment, the filtering device includes an input waveguide for directing optical signals to the photonic band structure region and an output waveguide for receiving optical signals from the photonic band structure region. Preferably, the device includes a first output waveguide and a second output waveguide, wherein the photonic band structure region is positioned between the input waveguide and the second output waveguide such that in use light passing to the first output waveguide from the input waveguide does not pass through the photonic bandgap region and light passing to the second output waveguide does pass through the photonic bandgap region, and is thereby filtered. Alternatively, the device can be arranged to couple light to the first output waveguide through the photonic bandgap region while light reflected from the photonic bandgap region is coupled to the second output waveguide. The waveguides may be ridge or rib type waveguides. As an example, the device may be used as an optical add-drop multiplexer (OADM). The photonic band structure region may include a defect in the vicinity of an output waveguide, the defect giving rise to a local defect passband within a bandgap, thereby in use allowing light at the defect wavelength to enter the output waveguide from the photonic band structure region.
As a further alternative, the optical device could be an optical coupler adapted to couple light diffracted by the photonic band structure region out of the plane of the waveguide structure to another optical device, preferably an optical fibre, positioned out of the plane of the waveguide structure.
In another implementation of the present invention, the optical device is an optical fibre comprising a central core surrounded by a Fibonacci spiral pattern of sub-regions which extend along at least a portion of the length of the optical fibre. This structure again gives rise to an isotropic photonic bandgap covering the wavelengths of operation of the optical fibre, thus confining optical signals carried by the optical fibre to the core region of the optical fibre.
According to a second aspect of the present invention, a method of processing an optical signal comprises the step of coupling an optical signal into an optical device comprising a waveguide structure having a photonic band structure region, the photonic band structure region including a first region having a first refractive index and an array of sub-regions having a second refractive index, the array of sub-regions being arranged in a Fibonacci spiral pattern.
According to a third aspect of the present invention, a method of manufacturing an optical device, comprises the steps of forming a photonic band structure region in a waveguide structure, the photonic band structure region including a first region having a first refractive index and an array of sub-regions having a second refractive index, the array of sub-regions being arranged in a Fibonacci spiral pattern.
The method may be a method of manufacturing an optical fibre comprising the steps of stacking tubes or rods of silica glass or polymers or plastics in a Fibonacci spiral pattern to form a preform, fusing the tubes or rods together and drawing the preform down in size in a fibre pulling tower.
Preferably, the tubes or rods are placed in a template or holder to hold them in the Fibonacci spiral pattern. Alternatively, the tubes or rods may be of different diameters allowing them to pack in a Fibonacci spiral pattern. In another alternative, the tubes or rods may be trapezoidal columns with a hole formed through them longitudinally, which stack together to form a Fibonacci spiral pattern of holes.
The method may also be a method of manufacturing an optical fibre comprising the steps of drilling a Fibonacci spiral pattern of holes in a block and drawing the block into a fibre.